Add %precedence support.

Unfortunately it is not possible to reuse the %prec directive.  This
is because to please POSIX, we do not require to end the rules with a
semicolon.  As a result,

foo: bar %prec baz

is ambiguous: either a rule which precedence is that of baz, or a rule,
and then a declaration of the precedence of the token baz.

	* doc/bison.texinfo: Document %precedence.
	(Precedence Only): New.
	* src/assoc.h, src/assoc.c (precedence_assoc): New.
	* src/conflicts.c (resolve_sr_conflict): Support it.
	* src/scan-gram.l, src/parse-gram.y (%precedence): New token.
	Parse it.
	* tests/calc.at: Use %precedence for NEG.
	* tests/conflicts.at (%precedence does not suffice)
	(%precedence suffices): New tests.

src/assoc.h

@@ -1,5 +1,5 @@
 /* Associativity information.
-   Copyright (C) 2002, 2006 Free Software Foundation, Inc.
+   Copyright (C) 2002, 2006, 2008 Free Software Foundation, Inc.
 
    This file is part of Bison, the GNU Compiler Compiler.
 
@@ -22,10 +22,11 @@
 /* Associativity values for tokens and rules.  */
 typedef enum
 {
-  undef_assoc,
-  right_assoc,
-  left_assoc,
-  non_assoc
+  undef_assoc,     /** Not defined. */
+  right_assoc,     /** %right */
+  left_assoc,      /** %left */
+  non_assoc,       /** %nonassoc */
+  precedence_assoc /** %precedence */
 } assoc;
 
 char const *assoc_to_string (assoc a);